1. Field of the Invention
This invention is directed to solid electrolytes containing a polymer matrix and an electrolyte solvent (plasticizer) for the polymer matrix. In particular, this invention is directed to solid electrolytes which include an inorganic ion salt, an oxyalkylene-containing solvent, and/or oxyalkylene-containing repeating units in the polymer matrix.
This invention is further directed to solid electrolytic cells (batteries) containing an anode, a cathode and a solid electrolyte containing a polymer matrix which includes an inorganic ion salt, an oxyalkylene-containing solvent and/or oxyalkylene-containing repeating units in the polymer matrix.
2. State of the Art
Electrolytic cells containing an anode, a cathode and a solid, solvent-containing electrolyte incorporating an inorganic ion salt are known in the art and are usually referred to as "solid batteries". These cells offer a number of advantages over electrolytic cells containing a liquid electrolyte (i.e., "liquid batteries") including improved safety features.
Specifically, solid batteries employ a solid electrolyte interposed between a cathode and an anode. The solid electrolyte contains either an inorganic or an organic matrix and a suitable inorganic ion salt as a separate component. The inorganic matrix may be non-polymeric [e.g., .beta.-alumina, silver oxide, lithium iodide, etc.] or polymeric [e.g., inorganic (polyphosphazene) polymers] whereas the organic matrix is typically polymeric. Suitable organic polymeric matrices are well known in the art and are typically organic polymers obtained by polymerization of a suitable organic monomer as described, for example, in U.S. Pat. No. 4,908,283. Suitable organic monomers include, by way of example, ethylene oxide, propylene oxide, ethyleneimine, epichlorohydrin, ethylene succinate, and an acryloyl-derivatized alkylene oxide containing an acryloyl group of the formula CH.sub.2 .dbd.CR'C(O)O-- where R' is hydrogen or lower alkyl of from 1-6 carbon atoms. Reference is made to Fiona M. Gray, "Solid Polymer Electrolytes", VCH Publishers, Inc., New York, N.Y., 1991, the disclosure of which is incorporated herein in its entirety.
Because of their expense and difficulty in forming into a variety of shapes, inorganic non-polymeric matrices are generally not preferred and the art typically employs a solid electrolyte containing a polymeric matrix. Nevertheless, electrolytic cells containing a solid electrolyte containing a polymeric matrix suffer from low ion conductivity and, accordingly, in order to maximize the conductivity of these materials, the matrix is generally constructed into a very thin film, i.e., on the order of about 25 to about 250 .mu.m. As is apparent, the reduced thickness of the film reduces the total amount of internal resistance within the electrolyte thereby minimizing losses in conductivity due to internal resistance.
The solid electrolytes also contain a solvent (plasticizer) which is added to the matrix primarily in order to enhance the solubility of the inorganic ion salt in the solid electrolyte and thereby increase the conductivity of the electrolytic cell. In this regard, the solvent requirements of the solvent used in the solid electrolyte have been art recognized to be different from the solvent requirements in liquid electrolytes. For example, solid electrolytes require a lower solvent volatility as compared to the solvent volatilities permitted in liquid electrolytes.
Suitable prior art solvents well known in the art for use in such solid electrolytes include, by way of example, propylene carbonate, ethylene carbonate, .gamma.-butyrolactone, tetrahydrofuran, glyme (dimethoxyethane), diglyme, tetraglyme, dimethylsulfoxide, dioxolane, sulfolane and the like.
Heretofore, the solid, solvent-containing electrolyte has typically been formed by one of two methods. In one method, the solid matrix is first formed and then a requisite amount of this material is dissolved in a volatile solvent. Requisite amounts of the inorganic ion salt and the electrolyte solvent (usually a glyme and an organic carbonate) are then added to the solution. This solution is then placed on the surface of a suitable substrate (e.g., the surface of a cathode) and the volatile solvent is removed to provide for the solid electrolyte.
In the other method, a monomer or partial polymer of the polymeric matrix to be formed is combined with appropriate amounts of the inorganic ion salt and the solvent. This mixture is then placed on the surface of a suitable substrate (e.g., the surface of the cathode) and the monomer is polymerized or cured (or the partial polymer is then further polymerized or cured) by conventional techniques (heat, ultraviolet radiation, electron beams, etc.) so as to form the solid, solvent-containing electrolyte.
When the solid electrolyte is formed on a cathodic surface, an anodic material can then be laminated onto the solid electrolyte to form a solid battery (i.e., an electrolytic cell).
Regardless of which of the above techniques is used in preparing the solid electrolyte, an important concern is the ability of the electrolyte solvent to dissolve the inorganic ion salt. In the constant search for improved solubility, the art has turned to solvent mixtures, the most widely used being the above-mentioned organic carbonate/glyme. The combination of carbonate and ether functions provides adequate solvation of the inorganic ion salt at relatively high salt concentration levels.
However, the use of carbonate/glyme or other solvent mixtures adds to the cost of producing the electrolyte, due to the necessity of storing separate inventories of solvents, and the need for mixing equipment to obtain the proper ratio of the solvents.